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Kinesthetic Displays for Remote and Virtual Environments B. Hannaford and S. Venema Summarized by Geb Thomas The Sense of Touch • Kinesthetic sense – movement or force in muscles and joints • Tactile sense – Nerves in skin for shapes and textures History • Direct Mechanical Systems (Goertz) • Then remote, position controlled robots (which worked poorly) Characteristics of Kinesthetic Channel • Two roles – Body position sense – Sensing and controlling contact with external environment • Bidirectional flow of energy • Rate of change of energy is: – Power = force*velocity Position/Force Simultaneity • Force Feedback – Sense velocity (and/or position); apply force • Can’t control both force and velocity • To sense force, one would have to sense force in 3 directions (x, y, z) and 3 torques (roll, pitch, yaw) Simulation • 2nd order linear system • Soft surfaces • Hard surfaces Hard Surfaces • Update rate determines realism • Bandwidth depends on the operator and the contacted object. • At least in Audio frequencies • From other references, 1kHz is cited as a reasonable value Physiology • Muscle is not a pure force generator or velocity generator. • Muscle spindles transduce muscle stretch and rate of stretch – Nonlinear – Principle source of body position information – Can be artificially stimulated with vibration • Golgi tendon organs encode muscle force • “Efferent copy” also encodes muscle force Reference Frame • Vision and hearing are global reference frames • Kinesthetic sensation is perceived with respect to limbs -- body reference frame • Kinesthetic sensation is localized to the specific object Contact modeling • • • • • Bond-graph method of Network theory f1-z1(vp) - z2(vp) - f2 = 0 => fz-z1(vp) - f2 + z2(vp) = fp Operator and display are equally important Can only control either force or velocity Force feedback display • Sense velocity, apply force – – – – uninhibited movement accurately reproduce force provide large forces for hard surfaces high bandwidth • Same requirements for robot manipulators for contact force control Displacement Feedback Displays • Sense force, impose controlled movement – – – – Rigid enough to block movement accurately reproduce displacements provide for free movement high bandwidth • Similar to robot manipulators for accurate trajectory following • More expensive because force sensors are expensive, position actuators are not available Cross Modality Displays • Keeps feedback as information • Extra cognitive burden Brakes • Exploratory • Constrain velocity to zero • Impossible to simulate contact with a surface not aligned with the main axes Design Issues • Kinematics – Must be in constant contact with a moving operator – Share a common ground – Denavit-Hartenberg notation • Degrees of Freedom – Range of motion – Complexity Singularity Analysis • One or more joints is at a motion limit – Workspace boundary singularity • Two or more joint axes become parallel – Workspace interior singularity • Jacobian matrix relates joint velocities (theta) to Cartesian velocities (V) – V = J(theta)* d(theta) – At singularities J become undefined and – d(theta) = J-1(theta)*V makes angle velocities go towards infinity Dynamics • Fo = Fa - Ff(x,v) - M d(v) • Mass interferes with display velocity – Particularly complex with multidimensional systems More Dynamics • Friction – Absorbs output force – Three types of friction • Static friction, resists onset of motion • Colomb, constant resistance to motion • Viscous, resists motion in proportion to velocity – Particularly important for slow motions • Stiffness – How the mechanism deforms under loads Examples - Salibury, JPL Examples -- Utah Future Challenges • Hard Contact problem • Real-time dynamic modeling • Mechanism design